The physical behavior and health of an alluvial stream depends on the extent of man's interference in its tributary areas. In its natural setting, a stream provides three main functions of: (1) allowing the conveyance of flood waters and drainage; (2) permitting the transportation of bed load, as well as suspended sediment load; and (3) assimilating some of the pollutants generated in its tributary areas. When a development is completed in a watershed which discharges to a stream, peak flood discharges and pollutant levels will be increased, while sediment yield will be reduced. If these increases and reduction are beyond the tolerance limit of the stream, it will be subjected to environmental and economic damages.
In order to prevent these damages, the following objectives must be achieved:
1. Reduce the peak flood discharge in order to prevent flooding along the downstream channels and to achieve a balance between water discharge and bed load discharge.
2. Allow the delivery of bed load (sand and gravel) contributed by the upstream watershed, thereby minimizing potential erosion and degradation along the downstream alluvial channel.
3. Minimize the discharge of pollutants to the downstream reach in order to preserve its biological, environmental and recreational values.
In the past, the general belief was that erosion and degradation of alluvial channels occur because of increases in peak discharge caused by the upstream development. Most cities and counties have adopted the popular drainage ordinance that states "Post-development discharges must not exceed the pre-development or natural discharges." This requirement is usually satisfied by constructing a local or regional detention basin. The term "detention" means holding excess flood water for a time in order to control the rate of flow to the downstream channel. Detention periods for basins located downstream of small watersheds (less than 100 acres) are usually a few hours and sometimes fractions of an hour.
In addition to controlling flood flows, conventional detention basins have provided retention of polluted water. The term "retention" means storing urban runoff that is of poor quality. Generally, the period of retention corresponds to several days or weeks until the retained water infiltrates to the ground or is lost through evaporation. If the retention basin is provided with an efficient filter system, the filtered water can be directly passed to the downstream reaches.
Frequently, conventional detention basins fail to achieve the objectives mentioned for the following reasons:
1. These basins not only retard floodwaters and capture pollutants, unfortunately they also trap the bed load so that none of it will reach the downstream alluvial channel. Even though the discharge may be reduced significantly, supply of inadequate quantities of bed load may cause erosion along the downstream reach.
2. In general, pollutants are adsorbed to the suspended load particles and not to the bed load particles. During the occurrence of a significant storm event when the overflow system of the basin becomes operational, some of the suspended load particles, and therefore pollutants, would escape from the basin.
3. Environmental constraints and costs make it difficult to locate suitable disposal sites for the accumulated sediments in the basin. In many city and county jurisdictions, sediment deposits are not removed for many years. The longer the deposits are kept in a basin, the less the storage capacity. This would reduce its detention capability, as well as its efficiency to capture suspended load particles. These poorly maintained basins not only continue to trap bed load, their design objectives, i.e., reduction in peak discharge and capture of pollutants, are not fulfilled.
4. Failure of basins to satisfy their objectives causes the downstream channels to generally show evidence of degradation. To prevent damage to structures and loss of property, the alluvial channel located downstream from a basin is paved or armored. This means loss of riparian habitat, which is no longer acceptable to our ecologically conscious society.
During construction periods, excessive sediment yield and delivery are usually controlled by constructing desilting basins. Frequently, those basins have inadequate sizes, permitting excessive discharge of suspended load which can be harmful to the downstream riparian habitat. Often, too many small desilting basins with little detention capability and efficiency to trap suspended sediments are constructed. Unfortunately, these basins trap most of the bed load. The combination of lack of attenuation and trapping significant quantities of bed load occasionally cause erosion along the alluvial reaches located downstream from the construction site. Fortunately, construction periods are short and channel degradation is a relatively slow process. During construction periods, usually the problems are reverse, i.e., common occurrence of aggradation as a result of excessive discharge of sediment loads to the downstream channels.